Code division multiple access (CDMA) communication technology can present many engineering design and verification challenges, due to complexities that can arise when integrating radio frequency (RF) and baseband designs to achieve a working system. Optimizing the design for a device incorporating CDMA technology can therefore be a process of compromise. The smallest and largest signals define the dynamic range of the receiver system. Noise limits the smallest signals that a receiver system is capable of processing. The largest signal is limited by distortions arising from nonlinearity of receiver circuits. Large interfering signals also hinder the reception of small desired signals. The nonlinearities in the receiver networks generate distortion products that fall within the receiver passband. These distortion products prohibit or reduce message reliability. An optimal noise design typically yields less than optimal large signal performance. The best large signal performance suffers from higher noise degradation, which in turn limits the weak signal reception. The error sources determine the bit error rate related to a particular transmitting power.
In addition to noise, a frequency error also significantly contributes to the bit error rate. Frequency error is caused by different oscillator frequencies at the transmitting and receiving portion of a communication device. The mismatch between the oscillator frequencies at the transmitting end and at the receiving end can be caused by manufacturing tolerances in the oscillators and oscillating crystals used.
In addition, the Doppler effect contributes to the frequency error. Relative movement between the transmitter and the receiver leads to a frequency shift in the signals transmitted.
A homodyne, direct conversion receiver, also referred to as a zero intermediate frequency (ZIF) receiver, translates a desired radio frequency (RF) frequency directly to baseband to recover information. Baseband is the range of frequencies occupied by the signal before modulation or after demodulation. The baseband frequencies are typically substantially below the RF frequencies. At low baseband frequencies, signals may approach or include direct current (DC). The upper frequency limit of baseband depends on the data rate, or speed, at which information is sent.
A performance parameter referred to as the second order intercept point (IP2) reflects a system's susceptibility to second-order distortion. The higher the value of input IP2, the higher a system's immunity to second-order interference for the targeted baseband signal. The IP2 is a key indicator of the receiver's behavior in the presence of a very strong amplitude modulated (AM) jammer signal relative to the receive signal. When IP2 is too low, an off-channel jammer signal will interfere with the receiver's operation because second order distortion will lead to an unwanted baseband signal, which will interfere with the desired baseband information.
DC offset has also presented a serious design challenge to ZIF receiver design. Ideally, only undistorted information results from down-converting an RF signal to baseband. The circuit mismatch inherent in both RF and baseband analog circuits typically introduces a DC error, however, which is then added to the baseband signal. This offset error can be affected by both temperature and time.
Unlike heterodyne designs, ZIF receivers also place severe restrictions on local oscillator (LO) leakage and reradiation. Since both the LO and RF receive channels operate on the same frequency, any LO reverse leakage from the mixer will travel backwards to the antenna from where it is radiated into the RF passband, causing potential interference to other spectrum users.
As improvements in wireless communications are continuously being sought, specifications are often still evolving when the design cycle starts. The system design engineer must be able to perform system tradeoffs, define RF analog and digital baseband subsystem requirements, and ensure that the design will work when it is finally put together. This can be a difficult design and verification challenge, particularly when RF analog and digital baseband engineers can be two different design groups. In addition, it is often important for RF analog and baseband designs to progress in parallel for faster time-to-market, making system performance interactions and potential problems between RF analog and baseband sections more difficult to verify and fix until prototype designs are completed and tested together. This highlights the need to provide design and verification capability that allows performance tradeoffs to be examined for RF and baseband sections separately, as well as together.